Under consideration means the editor hasn't read it yet. Under review means it is being peer-reviewed. Once a paper is submitted to a journal, it is assigned to a handling editor who conducts an initial screening of the paper to see if it meets the basic requirements of the journal.
I agree with Gaurav H Tandon that the editor has not read it and at this stage, it is being checked whether it is acceptable - such as the format, for example. Some journals have handling editors who undertake this role, some have an editorial office/manager/assistant/administrator for this role.